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LibGfx/WebPLoader: Relax vp8x / vp8l alpha check

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The check from 0805319d1e is too strict for animated images: It's
possible that an animation has alpha, but a frame doesn't.

(For example, if the animation is 10x10 pixels, but one frame updates
just 1x1 pixels. But even a full-sized frame could just not have alpha.)

If the VP8X header claims that the animation has alpha, we could check
that at least one frame has alpha, but for now we just don't do any
checking for animated images.
This commit is contained in:
Nico Weber 2024-05-27 20:30:08 -04:00 committed by Sam Atkins
parent c71b9fe6f8
commit 99d280a927
Notes: sideshowbarker 2024-07-17 08:25:15 +09:00 
Author: https://github.com/nico
Commit: https://github.com/SerenityOS/serenity/commit/99d280a927
Pull-request: https://github.com/SerenityOS/serenity/pull/24468
Reviewed-by: https://github.com/AtkinsSJ
Reviewed-by: https://github.com/LucasChollet ✅
1 changed files with 3 additions and 1 deletions
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4
Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp
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@ -518,7 +518,9 @@ static ErrorOr<NonnullRefPtr<Bitmap>> decode_webp_image_data(WebPLoadingContext&
VERIFY(!image_data.alpha_chunk.has_value());
auto vp8l_header = TRY(decode_webp_chunk_VP8L_header(image_data.image_data_chunk.data()));
if (context.first_chunk->id() == "VP8X" && context.vp8x_header.has_alpha != vp8l_header.is_alpha_used)
// Check that the VP8X header alpha flag matches the VP8L header alpha flag.
// FIXME: For animated images, if VP8X has alpha then at least one frame should have alpha. But we currently don't check this for animations.
if (context.first_chunk->id() == "VP8X" && !context.animation_frame_chunks_data.has_value() && context.vp8x_header.has_alpha != vp8l_header.is_alpha_used)
return Error::from_string_literal("WebPImageDecoderPlugin: VP8X header alpha flag doesn't match VP8L header");
return decode_webp_chunk_VP8L_contents(vp8l_header);